Block copolymers of the S-B-S type and processes for their preparation are well known in art for such uses as self-curing elastomers and as modifiers for diene elastomers. These block copolymers have a central core of a polybutadiene and have a block of polystyrene on each end. However, in view of the greater insolubility or non-dispersibility of the initial polystyrene, a solvent such as an aromatic hydrocarbon or cyclohexane has been used in its preparation.
This problem is recognized in British Patent No. 1,130,770 which states on page 2, lines 60-65, "In each of these processes, however, a vinyl aromatic hydrocarbon is required to be first polymerized and therefore the polymerization solvent used must be wholly or mainly a naphthenic or aromatic hydrocarbon solvent".
Similar recognition of this solubility problem is indicated in U.S. Pat. Nos. 3,231,635; 3,265,765 and 3,427,364 wherein statements are made regarding the necessity to use aromatic or naphthenic solvents or mixtures of these with other solvents.
British Patent No. 1,412,584 has modified the process of preparing block copolymers so that heptane may be used as the polymerization medium, by making first a block of t-butyl-styrene and describes this first block as a thermoplastic polymer of 10,000 to 100,000 molecular weight to give solubility to the growing block copolymer. The second or middle block of polybutadiene is then added and finally a block of polystyrene. The resulting block copolymer is poly(t-Bu-styrene)-polybutadiene-polystyrene.
U.S. Pat. No. 4,291,139 effects polymerization of block copolymers of the S-B-S type wherein S represents a block of polystyrene and B represents a block of polybutadiene using hexane as diluent by first preparing a "foot" or small block of homopolydiene to which the initial polystyrene block is thereafter attached. This small block of homopolydiene represents no more than 12 parts, preferably no more than 10 parts by weight of homopolydiene per 100 parts by weight of the block of polystyrene to which it is attached. Generally, therefore, this block has a molecular weight of 200-5000 depending on the size of the attached polystyrene block and is not large enough to alter the properties of the resultant S-B-S block copolymers except to give the initial polystyrene block greater dispersibility in hexane. This process is entirely conducted in a hexane solvent.
Because of its lower boiling point and resultant energy savings in the use of hexane as compared to benzene and cyclohexane, it is desirable to have a process for use in preparing S-B-S block copolymers wherein the final block copolymer is recoverable from a dispersing medium primarily containing low boiling alkanes such as hexane. Moreover, the use of a process primarily conducted in alkanes such as hexane makes the process adaptable for use in equipment designed for the preparation of polybutadiene in hexane.